Bipedal walking by common chimpanzees is known to differ both kinematically and kinetically from human bipedalism, in particular by the adoption of flexed-knee gaits and characteristically single-peaked vertical ground reaction force curves. This study provides a test of the proposition that the two former mechanical characteristics are functionally related. Further, it examines the pattern of forces occurring during normal human bipedal walking at slow, normal and fast velocities; and during walking imitating bipedal walking of the common chimpanzee, by both male and female human adults and children. The data suggest naturally reared chimpanzees walk with greater lateral forces than do adult humans walking erect. Women show greater variance in force parameters than do men; and children more than do adults, but men walk with greater vertical accelerations of the centre of mass, and sharper peaks of sagittal acceleration and deceleration than women. Data from both forceplate and pressure-pad records suggest that young children (and a chimpanzee for which forceplate data was available) accelerate using the heel, rather than the anterior part of the foot. A principal component analysis of Fourier coefficients for the force curves shows that forces produced by adults imitating chimpanzee gaits are intermediate between those produced in normal human gait and those produced by 'real' chimpanzees, confirming a functional connection between knee and hip kinematics during bipedal walking and the shape of the vertical ground reaction force curve. Human flexed-knee, flexed-hip walking is found to produce in-phase fluctuations in potential and kinetic energies, preventing the energy exchange which obtains in erect walking. Finally, computer simulations of bipedal walking indicate that kinematics are more influential determinants of ground reaction forces than are body parameters.